The present invention relates to a method and device for measuring the impact strength for individual granules over a wide distribution and for aspherical sizes.
In any manufacturing involving the use or movement of particulate materials, some breakdown or attrition of the particles is inevitable and has been reported in a wide range of processes and industries. For example, it has significance for those applications where it is desirable for particles to remain in a process almost indefinitely. The effects of attrition can be loss of product by removal of undersize particles, the need for recycling lost product, and the requirement for additional filtration. Another effect can be to limit the useful life of catalyst or enzyme particles.
Many products in the pharmaceutical industry, for example, are agglomerated granules which can suffer attrition during processing and also, if bulk packed, during shipment and use. Dust release into the atmosphere may be a hazard, but its release is also undesirable because of the high value of many of the products.
Attrition has a number of different effects, the relative importance of which is dependent upon the commercial or technical application. Properties of particulate materials change as a result of attrition. Loss of material occurs due to the change of particle sizes to smaller ones which are unacceptable to the particular process and which are removed from the process by accident or design in cyclones, filters, or precipitators. Wear of contaminant systems results from the impact of particles with the walls of the container or duct, and contamination of the process particles by debris from wear of the containment system may be significant in some applications. Even an explosion can be caused if a build-up of fine material is allowed to occur.
Several conventional methods for testing the mechanical strength of industrial catalysts which are used in fluid beds are reported in C. R. Bemrose and J. Bridgwater, A Review of Attrition and Attrition Test Methods, Powder Technology, 49 (1987) 97-126. One of the vibration tests discussed therein used a container enclosing a granular charcoal bed which was vibrated at 60 Hz with an acceleration of 5 g. Air was blown into the top of the container and dust formed by the attrition of the granular charcoal passed through the perforated base of the container to be collected on a glass fiber filter paper. As a result, the impact strength of the entire bed was tested and not individual particles.
Another vibration test is reported in T. P. Ponomareva, S. I Kontorovich, and E. D. Shchukin, Attrition Of Spherical Cracking Catalysts in the Presence Of Powdered Lubricants, Kinetics and Catalysis, 21 (1980) 505-510 for measuring the wear between catalyst particles treated with a lubricant powder. The test used a specially constructed cylindrical drum undergoing vertical vibrational movement imposed by a vibro-saw at a frequency of 50 Hz and an amplitude of 6 mm. Only the amount of wear between the catalyst particles themselves could be measured by removing the abrasion products through sieves located in the drum.
These methods have proven unsuitable for characterizing the individual particle since these methods vibrate an entire bed within a closed vessel. They measure the particle to vessel-wall interaction when the attrition and fragmentation of the particles is primarily caused by the particles rubbing each other. The impact velocity of the collisions encountered by the particle bed is also poorly defined by these methods and may be inaccurate due to some collisions encountering drag forces within the bed. Furthermore, some of these methods only measure spherical particles and are poorly adapted to accurately characterize aspherical or non-uniform shaped particles.
Thus, a need exists for a method and device for assessing attrition and fragmentation characteristics of particles during handling. A tool is needed to help develop particles which are individually strong since the mechanically stability of the particle is of primary importance in many industrial fields such as enzyme formulation technology. Characterizing the impact strength of individual particles, as opposed to entire particle bed, provides information that can be used to develop particles with enhanced attrition strength.
The present invention provides a test device for characterizing the impact strength of a granule. The test device includes a first container having an interior cavity larger than the size of the granule and means for vibrating the first container in a generally unidirectional movement at a predetermined frequency of resonance with sufficient strength to provide reproducible damage to the granule, the vibrating means connected to the first container.
The present invention also provides a test device for characterizing the impact strength of a granule which includes a vibrator providing a generally unidirectional movement at a predetermined frequency and a first container having an interior cavity larger than the size of the granule. The device further includes a first spring configured to connect at one end to the vibrator. The opposite end of the first spring is configured to connect to the container. The first spring has a predetermined frequency of resonance adapted to amplify the frequency of the vibrator movement and impart the amplification to the container.
A method of characterizing the impact strength of a granule is also provided by the present invention. The method includes the steps of: vibrating a sealed container having an interior space for confining a plurality of granules at a predetermined resonant frequency for a predetermined period of time and amplitude to cause repeated impact of the granules against the interior walls of the container; removing dust, fines and damaged granule fragments that do not exhibit a minimum desired size, from the container; and measuring the amount of undamaged granules and only slightly damaged granules that do exhibit the minimum desired size.
A method for characterizing the attrition rate of a granule is also provided by the present invention. The method includes the steps of: disposing granules to be tested within a closed container; repeatedly impacting the granules within the closed container such that granule damage occurs; removing dust, fines, and small granules and small granule fragments that do not exhibit a first minimum desired size; measuring the weight of undamaged granules and large damaged granules that exhibit the minimum desired size, as a consequence of a number of collisions between the granules and the container walls; again repeatedly impacting the undamaged granules and large damaged granules within the closed container such that granule damage occurs; removing dust, fines, and small granules and small granule fragments that do not exhibit a second minimum desired size; again measuring the weight of the undamaged granules and damaged granules that exhibit the second minimum desired size, as a consequence of a number of collisions with the container walls; and determining the attrition rate of the granules measured as the weight of dust generated per number of collisions or as a function of the weight of the granules relative to the original weight of the granules. Removing the dust, fines, small granules and small granule fragments that do not exhibit the desired minimum size can preferably be accomplished by sieving the impacted granules to separate the larger damaged and undamaged granules from the smaller dust, fines, fragments, and damaged granules.
Accordingly, it is an object of the present invention to provide a method and device for characterizing the impact strength of an individual granule.
Another object of the present invention is to provide a method and device which is adaptable to characterizing aspherical or non-uniformly shaped granules and accurately relates single granule to multiple granule results.
It is a further object of the present invention to provide a device which provides a higher number of well-controlled impacts for each granule compared to the prior art.
Still another object of the present invention is to provide a test device which is inexpensive to build and operate for a large number of representative samples to yield reproducible test results easily obtained without extensive operator training.
A further object of the present invention is to provide a test device which provides granules with collision orientations which are equally likely and minimizes drag force.
Other and further advantages, embodiments, variations and the like will be apparent to those skilled-in-the-art from the present specification taken with the accompanying figures and appended claims. The accompanying figures, which are incorporated in and constitute a part of this application, illustrate several exemplary embodiments of the present invention and together with description, serve to explain the principles of the present invention.